Field of the Invention
The present invention relates to an image sensor and an image capturing apparatus.
Description of the Related Art
In recent years, image sensors used in image capturing apparatuses, such as digital still cameras and digital video cameras, are becoming increasingly multifunctional.
Japanese Patent Laid-Open No. 2013-106194 discloses a technique to enable focus detection of a pupil-division type in an image sensor. According to Japanese Patent Laid-Open No. 2013-106194, each pixel composing the image sensor has two photodiodes that are configured to receive beams of light that have passed through different pupils of an image capture lens with the aid of one microlens. In this way, focus detection can be performed on the image capture lens by comparing signals output from the two photodiodes with each other, and a signal of a captured image can be obtained from a combined signal derived from the two photodiodes.
Japanese Patent Laid-Open No. 2013-106194 also discloses the following method: after reading out a reset level signal and a signal of a first photodiode, a signal of a second photodiode is combined without resetting, the resultant combined signal is read out, and the signal of the second photodiode is obtained by subtracting the signal of the first photodiode from the resultant combined signal. With this method, the reset level signal can be used in common for the signals of the first and second photodiodes, and thus readout of the reset level signal can be reduced by one session.
Meanwhile, Japanese Patent Laid-Open No. 2014-72541 discloses a method of obtaining a signal of a captured image by reading out signals of first and second photodiodes in parallel, applying A/D conversion to the signals, and then digitally combining the signals.
In the case of an image sensor in which two photodiodes are provided per pixel, in order to obtain a signal for focus detection and a signal for a captured image, it is necessary to acquire individual signals of the two photodiodes, as well as a combined signal derived therefrom. An image capturing apparatus described in Japanese Patent Laid-Open No. 2013-106194 reads out a combined signal for a captured image after reading out a signal of a first photodiode for focus detection, and hence does not allow for simultaneous acquisition of a signal for focus detection and a signal for a captured image. This has the risk of significantly increasing a readout time period compared to a conventional image sensor in which one photodiode is provided per pixel. On the other hand, an image capturing apparatus described in Japanese Patent Laid-Open No. 2014-72541 can obtain a signal for a captured image only if it executes combining processing after reading out signals of first and second photodiodes for focus detection.